Find.find -> Use Dir.children instead of Dir.entries

Dir.children is available since Feature #11302. Find.find can
use of the new list (having no '.' neither '..' entries), making
now superflous an if statement.

This change can improve the performance of Find.find when the path
has lots of entries (thousands?).

Some profiling I did using 50,000 files on a given folder, using this code:

total_size = 0
Find.find(ENV["HOME"]) do |path|
if FileTest.directory?(path)
if File.basename(path)[0] == ?.
Find.prune # Don't look any further into this directory.
else
next
end
else
total_size += FileTest.size(path)
end
end
end